Wireless communication networks such as Global System for Mobile communications (GSM) allow subscribers of one Public Land Mobile Network (PLMN) to roam into the coverage area of other PLMNs (for example in different countries) and make telephone calls while via those other networks. Historically in most cases two call ‘legs’ have been required to set up a roaming call for a pre-paid subscriber—one leg between the visited PLMN (VPLMN) and the home network, and a second call leg to the called party. In the case of post-paid subscribers the call may be routed back to the home network before being forwarded to the called party or it may go straight to the called party without traversing the home PLMN (HPLMN), unless of course the call is to another subscriber in the HPLMN.
Routing a mobile originated call back to the home network before forwarding it to the called party is expensive on resources. However a majority of mobile originated calls are to networks (wireline or mobile) other than the home network. So routing roaming calls via the home network uses a costly international trunk that may be unnecessary. A mobile subscriber roaming in a VPLMN in a foreign country and making a ‘local’ call within that country (for example to request a taxi) will nonetheless incur two international call legs. Until recently this inflexibility has been passed on to subscribers as high call charges for international roaming. However GSM networks are now starting to introduce roaming services facilitated by the use of Customised Applications for Mobile network Enhanced Logic (CAMEL). Various phases of CAMEL implementation are described in, for example, ETSI documents TS 101 285 (GSM 02.78) and TS 122 078 (3GPP TS 22.078).
CAMEL features allow the VPLMN switch handling the call request from the mobile station to suspend call processing after collecting the dialled digits and to contact the HPLMN for instructions on how to proceed with the call, for example using Service Switching Function (SSF) facilities associated with the switch. This interaction with the HPLMN is accomplished with messages sent via the Signalling System no.7 (SS7) signalling network, using CAMEL Application Part (CAP) protocol over the Transaction Capabilities Application Part (TCAP) protocol defined in the SS7 standards. This does not require a voice trunk. The HPLMN may then validate the call attempt (e.g. to confirm the subscriber has credit) and perform possible number translation (e.g. to convert short dial codes into telephone numbers) before instructing the originating VPLMN switch how to proceed with processing the call. For these purposes information will usually be extracted by a Service Control Function (SCF) entity in the HPLMN from databases in a Service Control Point (SCP) in the HPLMN's SS7 network. Typically the call will then be routed directly to the called party, avoiding any unnecessary call legs and trunk usage via the HPLMN. ‘Event detection points’ may be defined in the instructions from the HPLMN to cause event reports to be sent to the HPLMN during progress of the call, e.g. when the called party answers.
One advantageous characteristic for network operators of switching mobile roaming calls via the HPLMN for routing is that it is possible to assemble a call detail record (CDR) for each call instance by monitoring the SS7 signalling network links in the HPLMN and collecting ISDN User Part (ISUP) Message Signal Units (MSUs) relating to that call. These MSUs, such as the Initial Address Message, enable parameters of the call such as calling party, called party and duration to be readily correlated for billing and other purposes.
When CAMEL features are used the HPLMN will not have access to any ISUP MSUs for the call (unless the call is to another subscriber in the HPLMN). This means that an SS7 monitoring system cannot build CDRs for these mobile originated calls. Indeed no information about the existence of the call may be available to the HPLMN until billing records are exchanged between the two PLMNs. For example, although an SCP in the HPLMN's SS7 system will be aware of the existence of such calls as a result of its CAMEL protocol interactions with VPLMNs, this information is generally not available in real time to other HPLMN systems for fraud management, billing verification, call performance monitoring or customer care. Hence a way of assembling CDRs similar to those obtained from ISUP MSUs but actually derived otherwise, in order to feed data to these other systems, will be of value. An analogous requirement is envisaged to assemble CDRs for instances of usage of other telecommunications services, without access to signaling directly associated with implementing the service transport or delivery.